Act as a means to integrate JAXB 2.0 Class based callbacks with TopLink OXM
Listener based callbacks.

A Map of UnmarshalCallbacks are created during JAXB 2.0 Generation and are set on a
JAXBUnmarshallerListener on each JAXBUnmarshaller. These callbacks are used to invoke
the JAXB 2.0 Class Based callbacks on each object at the appropriate time.